Substance P in the nucleus of the solitary tract augments bronchopulmonary C fiber reflex output

Illuminating Airway Nerve Structure and Function in Chronic Cough

Airway nerves regulate vital airway functions including bronchoconstriction, cough, and control of respiration. Dysregulation of airway nerves underlies the development and manifestations of airway diseases such as chronic cough, where sensitization of neural pathways leads to excessive cough triggering. Nerves are heterogeneous in both expression and function. Recent advances in confocal imaging and in targeted genetic manipulation of airway nerves have expanded our ability to visualize neural organization, study neuro-immune interactions, and selectively modulate nerve activation. As a result, we have an unprecedented ability to quantitatively assess neural remodeling and its role in the development of airway disease. This review highlights our existing understanding of neural heterogeneity and how advances in methodology have illuminated airway nerve morphology and function in health and disease.

Prostaglandin E2 sensitizes the cough reflex centrally via EP3 receptor-dependent activation of NaV 1.8 channels

Background

Cough hypersensitivity is a major characteristic feature associated with several types of cough, including chronic cough, but its underlying mechanisms remain to be fully understood. Inflammatory mediators, such as prostaglandin E₂ (PGE₂), have been implicated in both peripheral induction and sensitization of the cough reflex. In this study, using a conscious guinea pig model of cough, we investigated whether PGE₂ can sensitize the cough reflex via central actions and, if so, via which mechanisms.

Methods

All drugs were administered by intracerebroventricular (i.c.v.) route and whole-body plethysmograph set-up was used for both induction, using aerosolized citric acid (0.2 M), and recording of cough. Immunohistochemistry was performed to confirm the expression of NaV 1.8 channels in the nucleus tractus solitarius (nTS).

Results

We show that both PGE₂ and the non-selective EP1/EP3 agonist, sulprostone, dose-dependently enhanced the citric acid-induced cough (P ≤ 0.001, P ≤ 0.01, respectively). Pretreatment with the EP1 antagonist, ONO-8130, did not affect the sulprostone-induced cough sensitization, whilst the EP3 antagonist, L-798,106, dose-dependently inhibited this effect (P ≤ 0.05). Furthermore, treatment with either the EP2 agonist, butaprost or the EP4 agonist, L-902,688, had no effect on cough sensitization. Additionally, pretreatment with either the TRPV1 antagonist, JNJ-17203212 or the TRPA1 antagonist, HC-030031, alone or in combination, nor with the NaV 1.1, 1.2, 1.3, 1.4, 1.6 and 1.7 channel blocker, tetrodotoxin, had any effect on the cough. In contrast, pretreatment with the NaV 1.8 antagonist, A-803467, dose-dependently inhibited this effect (P ≤ 0.05). Furthermore, NaV 1.8 channels were shown to be expressed in the nTS.

Conclusion

Collectively, our findings show that PGE₂ sensitizes the cough reflex centrally via EP3 receptor-dependent activation of NaV 1.8 but independently of TRPV1,TRPA1 and TTX-sensitive sodium channel activation. These results indicate that PGE₂ plays an important role in central sensitization of the cough reflex and suggest that central EP3 receptors and/or NaVv 1.8 channels may represent novel antitussive molecular targets.

Graphical Abstract

A role for neurokinin 1 receptor expressing neurons in the paratrigeminal nucleus in bradykinin-evoked cough in guinea-pigs

KEY POINTS

Airway projecting sensory neurons arising from the jugular vagal ganglia terminate centrally in the brainstem paratrigeminal nucleus, synapsing upon neurons expressing the neurokinin 1 receptor. This study aimed to assess the involvement of paratrigeminal neurokinin 1 receptor neurons in the regulation of cough, breathing and airway defensive responses. Lesioning neurokinin 1 receptor expressing paratrigeminal neurons significantly reduced cough evoked by inhaled bradykinin but not inhaled ATP or tracheal mechanical stimulation. The reduction in bradykinin-evoked cough was not accompanied by changes in baseline or evoked respiratory variables (e.g. frequency, volume or timing), animal avoidance behaviours or the laryngeal apnoea reflex. These findings warrant further investigations into targeting the jugular ganglia and paratrigeminal nucleus as a therapy for treating cough in disease.

ABSTRACT

Jugular vagal ganglia sensory neurons innervate the large airways and are thought to mediate cough and associated perceptions of airway irritations to a range of chemical irritants. The central terminals of jugular sensory neurons lie within the brainstem paratrigeminal nucleus, where postsynaptic neurons can be differentiated based on the absence or presence of the neurokinin 1 (NK1) receptor. Therefore, in the present study, we set out to test the hypothesis that NK1 receptor expressing paratrigeminal neurons play a role in cough evoked by inhaled chemical irritants. To test this, we performed selective neurotoxin lesions of NK1 receptor expressing neurons in the paratrigeminal nucleus in guinea-pigs using substance P conjugated to saporin (SSP-SAP). Sham lesion control or SSP-SAP lesion guinea-pigs received nebulised challenges, with the pan-nociceptor stimulant bradykinin or the nodose ganglia specific stimulant adenosine 5'-triphosphate (ATP), in conscious whole-body plethysmography to study cough and associated behaviours. Laryngeal apnoea reflexes and cough evoked by mechanical stimulation of the trachea were additionally investigated in anaesthetised guinea-pigs. SSP-SAP significantly and selectively reduced the number of NK1 receptor expressing neurons in the paratrigeminal nucleus. This was associated with a significant reduction in bradykinin-evoked cough, but not ATP-evoked cough, mechanical cough or laryngeal apnoeic responses. These data provide further evidence for a role of jugular vagal pathways in cough, and additionally suggest an involvement of NK1 receptor expressing neurons in the paratrigeminal nucleus. Therefore, this neural pathway may provide novel therapeutic opportunities to treat conditions of chronic cough.

Airway Innervation and Plasticity in Asthma

Airway nerves represent a mechanistically and therapeutically important aspect that requires better highlighting in the context of diseases such as asthma. Altered structure and function (plasticity) of afferent and efferent airway innervation can contribute to airway diseases. We describe established anatomy, current understanding of how plasticity occurs, and contributions of plasticity to asthma, focusing on target-derived growth factors (neurotrophins). Perspectives toward novel treatment strategies and future research are provided.

Modulation of protective reflex cough by acute immune driven inflammation of lower airways in anesthetized rabbits

Chronic irritating cough in patients with allergic disorders may reflect behavioral or reflex response that is inappropriately matched to the stimulus present in the respiratory tract. Such dysregulated response is likely caused by sensory nerve damage driven by allergic mediators leading to cough hypersensitivity. Some indirect findings suggest that even acid-sensitive, capsaicin-insensitive A-δ fibers called “cough receptors” that are likely responsible for protective reflex cough may be modulated through immune driven inflammation. The aim of this study was to find out whether protective reflex cough is altered during acute allergic airway inflammation in rabbits sensitized to ovalbumin. In order to evaluate the effect of such inflammation exclusively on protective reflex cough, C-fiber mediated cough was silenced using general anesthesia. Cough provocation using citric acid inhalation and mechanical stimulation of trachea was realized in 16 ovalbumin (OVA) sensitized, anesthetized and tracheotomised rabbits 24h after OVA (OVA group, n = 9) or saline challenge (control group, n = 7). Number of coughs provoked by citric acid inhalation did not differ between OVA and control group (12,2 ±6,1 vs. 17,9 ± 6,9; p = 0.5). Allergic airway inflammation induced significant modulation of cough threshold (CT) to mechanical stimulus. Mechanically induced cough reflex in OVA group was either up-regulated (subgroup named “responders” CT: 50 msec (50–50); n = 5 p = 0.003) or down-regulated (subgroup named “non responders”, CT: 1200 msec (1200–1200); n = 4 p = 0.001) when compared to control group (CT: 150 msec (75–525)). These results advocate that allergen may induce longer lasting changes of reflex cough pathway, leading to its up- or down-regulation. These findings may be of interest as they suggest that effective therapies for chronic cough in allergic patients should target sensitized component of both, reflex and behavioral cough.

A Double-Blind Randomized Controlled Trial to Determine the Preventive Effect of Hangekobokuto on Aspiration Pneumonia in Patients Undergoing Cardiovascular Surgery

PURPOSE

This study aimed to assess whether hangekobokuto (HKT) can prevent aspiration pneumonia in patients undergoing cardiovascular surgery.

METHODS

We performed a single-center, double-blinded, randomized, placebo-controlled study of HKT in patients undergoing cardiovascular surgery. JPS HKT extract granule (JPS-16) was used as HKT. The primary endpoint was defined as the prevention of postoperative aspiration pneumonia. The secondary endpoints included complete recovery from swallowing and coughing disorders.

RESULTS

Between August 2014 and August 2015, a total of 34 patients were registered in this study. The rate of subjects with postoperative aspiration pneumonia was significantly lower in the HKT group than in the placebo group (p = 0.017). In high-risk patients for aspiration pneumonia, the rate was significantly lower in the HKT group than in the placebo group (p = 0.015). The rate of subjects with swallowing disorders tended to be lower in the HKT group than in the placebo group (p = 0.091), and in high-risk patients, the rate was significantly lower in the HKT group than in the placebo group (p = 0.038).

CONCLUSIONS

HKT can prevent aspiration pneumonia in patients undergoing cardiovascular surgery. In high-risk patients for aspiration pneumonia, HKT can prevent aspiration pneumonia and improve swallowing disorders.

Emerging targets for cough therapies; NK1 receptor antagonists

Cough is mediated by vagal afferent fibres innervating the larynx and proximal airways. Pre-clinical studies suggest that vagal C fibres produce Substance P, one of the tachykinin family of neuropeptides, which has been shown to enhance cough via the neurokinin-1 (NK-1) receptor and studies in animal models have also shown that NK-1 antagonists are effective at blocking induced cough. In the past, tachykinin receptor antagonists have yielded disappointing results in treating asthma and cough, however most of the activity of the agents tested was restricted to the peripheral nervous system and also the outcomes measures evaluating cough not optimal. More recently a small proof of concept study has suggest that aprepitant, an NK-1 antagonist licensed for the prevention of chemotherapy induced nausea and vomiting, might have beneficial effects on cough frequency in patients with lung cancer. In this review we investigate the current evidence for the anti-tussive effect of these therapies and the clinical trials in progress.

Pilot study for risk assessment of aspiration pneumonia based on oral bacteria levels and serum biomarkers

Background

Aspiration pneumonia is a serious problem among elderly patients; it is caused by many risk factors including dysphagia, poor oral hygiene, malnutrition, and sedative medications. The aim of this study was to define a convenient procedure to objectively evaluate the risk of aspiration pneumonia in the clinical setting.

Methods

This prospective study included an aspiration pneumonia (AP) group, a community-acquired pneumonia (CAP) group, and a control (Con) group (patients hospitalized for lung cancer chemotherapy). We used the Oral Health Assessment Tool (OHAT), which assesses oral hygiene, and evaluated performance status, body mass index, serum albumin levels, substance P values in plasma, and oral bacterial counts.

Results

The oral health as assessed by the OHAT of the aspiration pneumonia group was significantly impaired compared with that of the CAP group and the control (5.13 ± 0.18, 4.40 ± 0.26, 3.90 ± 0.22, respectively; p < 0.05). The oral bacterial count in the aspiration pneumonia group (7.20 ± 0.11) was significantly higher than that in the CAP group (6.89 ± 0.12), consistent with the OHAT scores. Oral bacterial count was significantly reduced by oral care.

Conclusions

OHAT and oral bacterial counts can be a tool to assess the requirement of taking oral care and other preventive procedures in patients at high risk of aspiration pneumonia.

Vagal Afferent Processing by the Paratrigeminal Nucleus

The paratrigeminal nucleus is an obscure region in the dorsal lateral medulla, which has been best characterized as a collection of interstitial cells located in the dorsal tip of the spinal trigeminal tract. The paratrigeminal nucleus receives afferent input from the vagus, trigeminal, spinal, and glossopharyngeal nerves, which contribute to its long-known roles in the baroreceptor reflex and nociceptive processing. More recently, studies have shown that this region is also involved in the processing of airway-derived sensory information. Notably, these studies highlight an underappreciated complexity in the neuronal content and circuit connectivity of the paratrigeminal nucleus. However, much remains to be understood about how paratrigeminal processing of vagal afferents is altered in disease. The aim of the present review is to provide an update of the current understanding of vagal afferent processing in the paratrigeminal nucleus and to explore how dysregulation at this site may contribute to vagal sensory neural dysfunction during disease.

The Neurokinin-1 Receptor Antagonist Orvepitant Is a Novel Antitussive Therapy for Chronic Refractory Cough: Results From a Phase 2 Pilot Study (VOLCANO-1)

BACKGROUND

Substance P and the neurokinin-1 (NK-1) receptor are implicated in chronic refractory cough pathophysiology. We assessed the efficacy and safety of orvepitant, a brain-penetrant NK-1 antagonist, in an open-label study in CRC patients with chronic refractory cough.

METHODS

Thirteen patients with daytime cough frequency >3 to <250 coughs/h took orvepitant 30 mg once daily for 4 weeks. Objective cough frequency was measured over 24 h at baseline and weeks 1, 4, and 8. The primary end point was change from Baseline in daytime cough frequency at week 4. Secondary end points included cough severity visual analog scale (VAS) score, global ratings of change for cough frequency and severity, and Cough-specific Quality of Life Questionnaire score.

RESULTS

All patients completed the study. Mean baseline cough frequency was 71.4/h. A statistically and clinically significant improvement in objective daytime cough frequency was observed at week 4: reduction from baseline of 18.9 (26%) coughs/h (95% CI, 9.6-28.3; P < .001). This effect was apparent at week 1 (reduction from baseline of 27.0 [38%] coughs/h [95% CI, 11.4-42.7; P = .001]) and sustained after drug discontinuation at week 8 (reduction from baseline of 20.4 [29%] coughs/h [95% CI, 3.2-37.5; P = .020]). Statistically significant improvements were seen for severity VAS and quality of life. Orvepitant was safe and well-tolerated.

CONCLUSIONS

Orvepitant resulted in a significant and sustained improvement in objective cough frequency, severity VAS, and quality of life; appeared safe; and merits further clinical investigation.

TRIAL REGISTRY

EU Clinical Trials Register; No.: 2014-003947-36; URL: www.clinicaltrialsregister.eu.

Dorsal Vagal Complex Modulates Neurogenic Airway Inflammation in a Guinea Pig Model With Esophageal Perfusion of HCl

Neurogenic airway inflammation in chronic cough and bronchial asthma related to gastroesophageal reflux (GER) is involved in the esophageal–bronchial reflex, but it is unclear whether this reflex is mediated by central neurons. This study aimed to investigate the regulatory effects of the dorsal vagal complex (DVC) on airway inflammation induced by the esophageal perfusion of hydrochloric acid (HCl) following the microinjection of nuclei in the DVC in guinea pigs. Airway inflammation was evaluated by measuring the extravasation of Evans blue dye (EBD) and substance P (SP) expression in the airway. Neuronal activity was indicated by Fos expression in the DVC. The neural pathways from the lower esophagus to the DVC and the DVC to the airway were identified using DiI tracing and pseudorabies virus Bartha (PRV-Bartha) retrograde tracing, respectively. HCl perfusion significantly increased plasma extravasation, SP expression in the trachea, and the expression of SP and Fos in the medulla oblongata nuclei, including the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMV). The microinjection of glutamic acid (Glu) or exogenous SP to enhance neuronal activity in the DVC significantly potentiated plasma extravasation and SP release induced by intra-esophageal perfusion. The microinjection of γ-aminobutyric acid (GABA), lidocaine to inhibit neuronal activity or anti-SP serum in the DVC alleviated plasma extravasation and SP release. In conclusion, airway inflammation induced by the esophageal perfusion of HCl is regulated by DVC. This study provides new insight for the mechanism of airway neurogenic inflammation related to GER.

TRPA1 mediates the cardiac effects of acrolein through parasympathetic dominance but also sympathetic modulation in mice

Numerous studies have demonstrated that short-term air pollution exposure causes cardiac autonomic imbalance as measured by heart rate variability (HRV). We previously showed that a single exposure to acrolein, a ubiquitous gaseous component of air pollution, not only causes autonomic imbalance, but also increases arrhythmia through transient receptor potential A1 (TRPA1) cation channels. Thus, the goal of this study was to characterize acrolein-induced autonomic changes in both normal and TRPA1-knockout mice (KO). Conscious, unrestrained C57BL/6 (WT) and KO mice were exposed to 3 ppm acrolein for 3 h. Separate groups were treated with either atenolol (sympathetic blocker), atropine (parasympathetic blocker) or hexamethonium (autonomic neurotransmission blocker), immediately before exposure. Electrocardiogram (ECG) and heart rate (HR) were recorded continuously before, during and after exposure. Exposure to acrolein produced significant increases in standard deviation of normal-to-normal R-R intervals (SDNN), Root Mean Square of the Successive Differences (RMSSD) and Low-Frequency (LF), as well as an increase in arrhythmia in WT mice. Treatment with atenolol reduced this response while atropine enhanced it, and both drugs blocked the acrolein-induced increase in arrhythmia; hexamethonium had no effect. On the other hand, neither acrolein nor any drug had an effect in the KO mice. Thus, acrolein-induced HRV responses appear to be mediated by a combined parasympathetic and sympathetic modulation. KO mice did not demonstrate any increases in HRV with exposure to acrolein. These data demonstrate that the cardiac effects of irritant air pollutants likely involve disruption of homeostatic balance and altered regulation even in healthy animals.

The research of the possible mechanism and the treatment for capsaicin-induced cough

Unexplained chronic cough (UCC) affects millions of patients worldwide. New therapeutic approaches to this condition are urgently needed, since current treatment options provide only symptomatic relief. Cough reflex hypersensitivity has been shown to play an important role in the pathogenesis of UCC. The transient receptor potential vanilloid type 1 (TRPV1) is present on peripheral terminals of airway sensory nerves and modulation of its activity represents a potential target for the pharmacological treatment of UCC. The aim of this study was to explore the efficacy and the possible mechanism of SB705498, a TRPV1 antagonist, for cough in a capsaicin-induced cough animal model (i.e. guinea pigs). Induction of cough by capsaicin was successfully implemented in the guinea pigs, and the animals that met the inclusion criteria were randomly divided into four treatment groups: (1) Saline inhalation group (NSInh group, N = 10, negative control group), (2) Codeine phosphate intraperitoneal injection group (CPInp group, N = 10, positive control group), (3) SB705498 inhalation group (SBInh group, N = 10), (4) SB705498 intragastric administration group (SBIng group, N = 10). After treatment with above compounds, the capsaicin-induced cough experiment was performed again. The cough numbers and the cough incubation periods were recorded to evaluate the antitussive effect of SB705498. Enzyme-linked immunosorbent assay (ELISA) testing and Immunohistochemistry (IHC) staining for substance P (SP), calcitonin gene related peptide (CGRP) and neurokinin A (NKA) expression in lung and brain tissues were performed as an indication of neurogenic inflammation. Hematoxylin-Eosin (H&E) staining was used to observe the pathology morphology of lung and brain tissues. When the CPInp, SBInh and SBIng groups were compared to the NSInh group, the cough numbers were significantly reduced (p < .001), while the cough incubation periods were significantly prolonged (P < .001). In addition, the expression of SP, CGRP and NKA in lung and brain tissue was reduced (P < .05). None of the animals in the four groups exhibited lung and brain parenchymal inflammation. The results from this study showed that SB705498 had a significant antitussive effect, could reduce the neurogenic inflammation by reducing the expression of SP, CGRP and NKA in a capsaicin-induced cough model of guinea pigs. The results further indicated that TRPV1 played an important role in UCC and SB705498 might be a promising therapeutic agent for UCC.

Vagotomy Decreases the Neuronal Activities of Medulla Oblongata and Alleviates Neurogenic Inflammation of Airways Induced by Repeated Intra-Esophageal Instillation of HCl in Guinea Pigs

Neuronal activity in the medulla oblongata and neurogenic inflammation of airways were investigated in a guinea pig model induced by repeated intra-esophageal instillation of hydrochloric acid (HCl) after vagotomy. Unilateral vagotomy was performed in the vagotomy group, while a sham-operation was performed in the sham group. Operation was not conducted in sham control group. Airway inflammation was observed with hematoxylin and eosin (HE) staining. C-fos protein was measured by immunohistochemistry (IHC) and Western blot (WB). Substance P was examined by IHC and enzyme-linked immuno sorbent assay (ELISA). Airway microvascular permeability was detected by evans blue dye (EBD) fluorescence. Inflammation of airway was observed in the trachea and bronchi after chronic HCl perfusion into the lower esophagus, and was alleviated after unilateral vagotomy. C-fos expression in the medulla oblongata was lower in the vagotomy group compared to the sham control and sham groups. Substance P-like immunoreactivity (SP-li), concentration and microvascular leakage in airway were lower in the vagotomy group than that in the other groups. Our results suggest that vagotomy improved neurogenic inflammation of airways and decreased neuronal activities, the afferent nerves and neurons in medulla oblongata may be involved in neurogenic inflammation of airways mediated by esophageal-bronchial reflex.

Prokinetic effects of neurokinin-2 receptor agonists on the bladder and rectum of rats with acute spinal cord transection

The suitability of various neurokinin-2 (NK2) receptor agonists and routes of administration to elicit on-demand voiding of the bladder and bowel, as future therapy for individuals with spinal cord injury, was examined using a rat model. The current study examined the feasibility of alternative routes of administration, which are more practical for clinical use than intravenous (IV) administration. Voiding and isovolumetric cystometry were recorded in anesthetized, acutely spinalized, female rats after IV, subcutaneous (SC), intramuscular (IM), intranasal (IN), or sublingual (SL) administration of [Lys⁵,MeLeu⁹,Nle¹⁰]-NKA_(4-10) (LMN-NKA). Administration of LMN-NKA (1-10μg/kg IV; 10-300μg/kg SC or IM; 15-1000μg/kg IN or 300-1500μg/kg SL) elicited rapid-onset, short-duration, dose-related increases in bladder pressure and voiding with the rank order for time of both onset and duration being IV < IN < SC = IM < SL. The incidence of voiding was dependent on the dose and route, with all routes resulting in a high voiding efficiency (~ 70%). Like LMN-NKA, neurokinin A (NKA 1-100μg/kg IV) and GR 64349 (0.1-30μg/kg IV or 1-300μg/kg SC) produced rapid-onset, short-duration increases in bladder pressure, as well as colorectal pressure. Administration of vehicle never produced bladder or rectal contractions or voiding. Transient hypotension was observed after IV injection of LMN-NKA, which was less pronounced after SC injection. Hypotension was not apparent with GR 64349. In conclusion, selective NK2 receptor agonists, administered through various non-IV routes of administration, may provide a safe, convenient, and efficacious method for inducing voiding.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Substance P/Neurokinin 1 and Trigeminal System: A Possible Link to the Pathogenesis in Sudden Perinatal Deaths

Sudden demise of a healthy fetus or a neonate is a very tragic episode in the life of parents. These deaths have been a mystery since ages but still remain unexplained. This review proposes the involvement of trigeminal nerve, neurotransmitter substance P (SP), and its receptor neurokinin 1 (NK-1R) in regulation of cardiorespiratory control in fetuses and newborns. Anomalies and immaturity of neuroregulatory systems such as trigeminal system in medulla oblongata of brainstem may provide a possible mechanism of sudden perinatal deaths. Vulnerable infants are born with respiratory center immaturity which in combination with any stressor such as cold, hypoxia, and smoking may lead to cessation of breathing and ventilatory response. SP/NK-1R may be involved in regulating the ventilatory control in neonates while it is decreased in fetal and adult life in humans, and any alterations from these may lead to irreversible sleep apnea and fatal breathing, ultimately sudden death. This review summarizes the studies performed to highlight the expression of SP or NK-1R in sudden perinatal deaths and proposes the involvement of trigeminal ganglion along with its nerve and SP/NK-1R expression alteration as one of the possible pathophysiological underlying mechanism. However, further studies are required to explore the role of SP, NK-1R, and trigeminal system in the pathogenesis of sudden infant deaths, sudden intrauterine deaths, stillbirths, and sudden deaths later in human life.

Etiology and treatment of cough in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive disease of dysregulated wound healing leading to unremitting scarring and loss of lung function. The predominant symptoms are dyspnea on exertion and a persistent dry cough. For patients with IPF, cough is more than just bothersome; it has a significant negative impact on quality of life and is a marker of disease severity and progression. The etiology of cough in IPF is unclear but may be due to architectural distortion of the lungs, increased sensitivity of the cough reflex, airway inflammation, or changes in mucus production and clearance. There also may be an overlap between IPF cough and cough due to other common etiologies such as asthma, gastroesophageal reflux disease, upper airway cough syndrome, and medications. There are no approved therapies to specifically treat IPF cough, and recently approved medications for IPF have not been evaluated in cough. Few clinical trials have focused on treatments for IPF cough. To date, there is only one randomized, placebo control therapeutic study for IPF cough with thalidomide, which significantly reduced IPF cough and improved quality of life. Two additional cohort studies report that interferon-α and prednisolone also decrease IPF cough. However, no medication is approved to treat IPF cough. Currently, the mainstay of therapy for IPF cough is standard cough suppressants, which have limited efficacy and often intolerable side effects. Future studies are needed to determine an effective therapy to alleviate this particularly debilitating symptom and improve overall quality of life for patients suffering with IPF.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Prenatal nicotinic exposure upregulates pulmonary C-fiber NK1R expression to prolong pulmonary C-fiber-mediated apneic response

Prenatal nicotinic exposure (PNE) prolongs bronchopulmonary C-fiber (PCF)-mediated apneic response to intra-atrial bolus injection of capsaicin in rat pups. The relevant mechanisms remain unclear. Pulmonary substance P and adenosine and their receptors (neurokinin-A receptor, NK1R and ADA1 receptor, ADA1R) and transient receptor potential cation channel subfamily V member 1 (TRPV1) expressed on PCFs are critical for PCF sensitization and/or activation. Here, we compared substance P and adenosine in BALF and NK1R, ADA1R, and TRPV1 expression in the nodose/jugular (N/J) ganglia (vagal pulmonary C-neurons retrogradely labeled) between Ctrl and PNE pups. We found that PNE failed to change BALF substance P and adenosine content, but significantly upregulated both mRNA and protein TRPV1 and NK1R in the N/J ganglia and only NK1R mRNA in pulmonary C-neurons. To define the role of NK1R in the PNE-induced PCF sensitization, the apneic response to capsaicin (i.v.) without or with pretreatment of SR140333 (a peripheral and selective NK1R antagonist) was compared and the prolonged apnea by PNE significantly shortened by SR140333. To clarify if the PNE-evoked responses depended on action of nicotinic acetylcholine receptors (nAChRs), particularly α7nAChR, mecamylamine or methyllycaconitine (a general nAChR or a selective α7nAChR antagonist) was administrated via another mini-pump over the PNE period. Mecamylamine or methyllycaconitine eliminated the PNE-evoked mRNA and protein responses. Our data suggest that PNE is able to elevate PCF NK1R expression via activation of nAChRs, especially α7nAChR, which likely contributes to sensitize PCFs and prolong the PCF-mediated apneic response to capsaicin.

Desflurane but not sevoflurane augments laryngeal C-fiber inputs to nucleus tractus solitarii neurons by activating transient receptor potential-A1

AIMS

Volatile anesthetics have distinct odors and some are irritating to the upper airway and may cause cough and laryngospasm, which may result, in part, from stimulation of C-fiber reflex. Local exposure of such anesthetics increases the sensitivity of capsaicin-sensitive laryngeal C-fiber endings compatible with airway irritability presumably by activation of transient receptor potential (TRP) ion channels, but the physiological relevance of this sensitization transmitted to the higher-order neurons in the central reflex pathway and output is unknown.

MAIN METHODS

In anesthetized young guinea pigs, baseline and left atrial capsaicin evoked changes in the extracellular unit activity of laryngeal C-fiber-activated neurons in the nucleus tractus solitarii (NTS) and phrenic nerve activity were compared between irritant (desflurane) and non-irritant (sevoflurane) anesthetic gas exposure to the isolated larynx.

KEY FINDINGS

Desflurane significantly augmented the peak and duration (p<0.01) of the NTS neuronal responses and the prolongation of expiratory time (p=0.017). The effect was enhanced by iontophoretic application of the TRPA1 agonist allyl-isothiocyanate (p<0.05), inhibited by TRPA1 antagonist HC-030031 (p<0.01), but not by TRPV1 antagonist BCTC. Sevoflurane did not affect the central pathway.

SIGNIFICANCE

Thus, the sensitization of the laryngeal C-fiber endings by irritant volatile anesthetics is transmitted to the NTS via activation of the TRPA1 and is associated with a prolonged reflexively evoked expiratory apnea. The findings may help to explain local deleterious effects of irritant volatile general anesthetics on the airways during inhaled induction or bronchodilator therapy for status asthmatics.

8-OH-DPAT abolishes the pulmonary C-fiber-mediated apneic response to fentanyl largely via acting on 5HT1A receptors in the nucleus tractus solitarius

Intravenous bolus injection of morphine causes a vagal-mediated brief apnea (∼3 s), while continuous injection, via action upon central μ-opioid receptor (MOR), arrests ventilation (>20 s) that is eliminated by stimulating central 5-hydroxytryptamine 1A receptors (5HT(1A)Rs). Bronchopulmonary C-fibers (PCFs) are essential for triggering a brief apnea, and their afferents terminate at the caudomedial region of the nucleus tractus solitarius (mNTS) that densely expresses 5HT(1A)Rs. Thus we asked whether the vagal-mediated apneic response to MOR agonists was PCF dependent, and if so, whether this apnea was abolished by systemic administration of 8-hydroxy-2-(di-n-propylamino)tetral (8-OH-DPAT) largely through action upon mNTS 5HT(1A)Rs. Right atrial bolus injection of fentanyl (5.0 μg/kg, a MOR agonist) was performed in the anesthetized and spontaneously breathing rats before and after: 1) selective blockade of PCFs' conduction and subsequent bivagotomy; 2) intravenous administration of 5HT(1A)R agonist 8-OH-DPAT; 3) intra-mNTS injection of 8-OH-DPAT; and 4) intra-mNTS injection of 5HT(1A)R antagonist WAY-100635 followed by 8-OH-DPAT (iv). We found the following: First, fentanyl evoked an immediate apnea (2.5 ± 0.4 s, ∼6-fold longer than the baseline expiratory duration, T(E)), which was abolished by either blocking PCFs' conduction or bivagotomy. Second, this apnea was prevented by systemic 8-OH-DPAT challenge. Third, intra-mNTS injection of 8-OH-DPAT greatly attenuated the apnea by 64%. Finally, intra-mNTS microinjection of WAY-100635 significantly attenuated (58%) the apneic blockade by 8-OH-DPAT (iv). We conclude that the vagal-mediated apneic response to MOR activation depends on PCFs, which is fully antagonized by systemic 8-OH-DPAT challenge largely via acting on mNTS 5HT(1A)Rs.

Perspective on the human cough reflex

This review dissects the complex human cough reflex and suggests hypotheses about the evolutionary basis for the reflex. A mechanosensory-induced cough reflex conveys through branches of myelinated Aδ nerve fibers is not chemically reactive (i.e., capsaicin, bradykinin); possibly, its evolution is to prevent the harmful effects of aspiration of gastric or particulate contents into the lungs. This became necessary as the larynx moves closer to the opening of the esophagus as human ancestors adapt phonation over olfaction beginning less than 10 million years ago. The second type of cough reflex, a chemosensory type, is carried by unmyelinated C fibers. Supposedly, its origin dates back when prehistoric humans began living in close proximity to each other and were at risk for infectious respiratory diseases or irritant-induced lung injury. The mechanism for the latter type of cough is analogous to induced pain after tissue injury; and, it is controlled by the identical transient receptor potential vanilloid cation channel (TRPV1). The airways do not normally manifest nociceptive pain from a stimulus but the only consistent response that capsaicin and lung inflammation provoke in healthy human airways is cough. TRPA1, another excitatory ion channel, has been referred to as the "irritant receptor" and its activation also induces cough. For both types of cough, the motor responses are identical and via coordinated, precisely-timed and sequential respiratory events orchestrated by complex neuromuscular networking of the diaphragm, chest and abdominal respiratory muscles, the glottis and parts of the brain.

Chronic intermittent hypoxia reduces neurokinin-1 (NK(1)) receptor density in small dendrites of non-catecholaminergic neurons in mouse nucleus tractus solitarius

Chronic intermittent hypoxia (CIH) is a frequent concomitant of sleep apnea, which can increase sympathetic nerve activity through mechanisms involving chemoreceptor inputs to the commissural nucleus of the solitary tract (cNTS). These chemosensory inputs co-store glutamate and substance P (SP), an endogenous ligand for neurokinin-1 (NK(1)) receptors. Acute hypoxia results in internalization of NK(1) receptors, suggesting that CIH also may affect the subcellular distribution of NK(1) receptors in subpopulations of cNTS neurons, some of which may express tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis (TH). To test this hypothesis, we examined dual immunolabeling for the NK(1) receptor and TH in the cNTS of male mice subjected to 10days or 35days of CIH or intermittent air. Electron microscopy revealed that NK(1) receptors and TH were almost exclusively localized within separate somatodendritic profiles in cNTS of control mice. In dendrites, immunogold particles identifying NK(1) receptors were prevalent in the cytoplasm and on the plasmalemmal surface. Compared with controls, CIH produced a significant region-specific decrease in the cytoplasmic (10 and 35days, P<0.05, unpaired Student t-test) and extrasynaptic plasmalemmal (35days, P<0.01, unpaired Student t-test) density of NK(1) immunogold particles exclusively in small (<0.1microm) dendrites without TH immunoreactivity. These results suggest that CIH produces a duration-dependent reduction in the availability of NK(1) receptors preferentially in small dendrites of non-catecholaminergic neurons in the cNTS. The implications of our findings are discussed with respect to their potential involvement in the slowly developing hypertension seen in sleep apnea patients.

Central mechanisms I: plasticity of central pathways

Cough is the most common symptom for which individuals seek medical attention and spend health-care dollars. Despite the burden induced by cough, the current treatments for cough are only partially effective. Delineating the sites and mechanisms in the cough central network for changes in the cough reflex could lead to new therapeutic strategies and drug target sites for more effective treatments. The first synaptic target in the CNS for the cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS); these neurons reorganize the primary sensory information into a coherent output. The NTS neurons have been shown to undergo neuroplasticity under a variety of conditions, such as respiratory disorders, stress, and exposures to environmental pollutants. The NTS contains a rich innervation of substance P immunoreactive nerve terminals, suggesting that substance P might be important in altered cough reflex response. This chapter summarizes our current findings on the role of substance P in enhanced cough reflex as well as the potential NTS targets for the action of substance P.

Clinical cough VI: the need for new therapies for cough: disease-specific and symptom-related antitussives

Cough is a common symptom that can be self-limiting or persistent. Ideally, treatment of the underlying cause(s) of cough with specific treatments should eliminate cough. This approach may not be successful if no cause can be established or if the treatment of the cause fails. Suppression of cough may be disease-specific or symptom-related. There has been a long tradition in acute cough usually due to upper respiratory tract infections to use symptom-related antitussives. In chronic cough, suppression of cough may be achieved by disease-specific therapies, but in many patients it may be necessary to use symptomatic antitussives. The efficacy of some over-the-counter symptomatic antitussives is often no better than that of a placebo. Currently available cough suppressants include the centrally acting opioids such as morphine, codeine, pholcodeine, and dextromethorphan. Early studies reported success in reducing cough in patients with chronic bronchitis or chronic obstructive pulmonary disease (COPD); however, a carefully conducted blinded controlled study showed no effect of codeine on cough of COPD. Success with these cough suppressants may be achieved at high doses that are associated with side effects. A slow-release preparation of morphine has been shown to have some degree of efficacy, but this should be reserved for the most severe chronic cough patient, and for patients with terminal cancer who may also benefit from its analgesic effects. There are case reports of the success of centrally acting drugs such as amitriptyline, paroxetine, gabapentin, and carbamezepine in chronic cough. New agents derived from basic research such as new opioids such as nociceptin or antagonists of transient receptor potential vanniloid-1 may turn out to have antitussive effects. Efficacy of symptomatic cough suppressants must be tested in double-blind randomized trials using validated measures of cough in patients with chronic cough not responding to specific treatments. Patients with chronic cough need effective antitussives that could be used either on demand or on a long-term basis.

Role of chemoreceptors in mediating dyspnea

Dyspnea, or the uncomfortable awareness of respiratory distress, is a common symptom experienced by most people at some point during their lifetime. It is commonly encountered in individuals with pulmonary disease, such as chronic obstructive pulmonary disease (COPD), but can also be seen in healthy individuals after strenuous exercise, at altitude or in response to psychological stress. Dyspnea is a multifactorial sensation involving the brainstem, cortex, and limbic system, as well as mechanoreceptors, irritant receptors and chemoreceptors. Chemoreceptors appear to contribute to the sensation of dyspnea in two ways. They stimulate the respiratory control system in response to hypoxia and/or hypercapnia, and the resultant increase respiratory motor output can be consciously perceived as unpleasant. They also can induce the sensation of dyspnea through an as yet undetermined mechanism-potentially via direct ascending connections to the limbic system and cortex. The goal of this article is to briefly review how changes in blood gases reach conscious awareness and how chemoreceptors are involved in dyspnea.

Peripheral mechanisms I: plasticity of peripheral pathways

Cough plays a vital role in protecting the lower airways from inhaled irritants, pollutants, and infectious agents. The cough reflex exhibits remarkable plasticity, such that in the context of infectious or inflammatory respiratory diseases such as asthma, chronic bronchitis, and idiopathic pulmonary fibrosis the cough reflex can become dysregulated, leading to a chronic cough. A chronic, nonproductive (dry) cough can rob sufferers of quality of life. Plasticity of the cough reflex likely involves multiple intersecting pathways within the airways, the peripheral nerves that supply them, and the central nervous system. While further studies are needed to determine the presence and relevance of many of these specific pathways in cough associated with chronic respiratory disease, the last decade has yielded unprecedented insight into the molecular identity of the ion channels and associated proteins that initiate and conduct action potentials in the primary sensory nerves involved in reflexes such as cough. We now know, for instance, that members of the transient receptor potential superfamily of nonselective cation channels function as transducers that convert specific external stimuli into neuronal activation. We also know that certain Na+ and K+ channels play specialized roles in regulating action potential discharge in irritant-sensing afferent nerves. In this chapter, we summarize the available information regarding factors that may modulate afferent neuron function acutely, via posttranslational modifications and over the longer term through neurotrophin-dependent alterations of the transcriptional programs of adult sensory neurons.

Effects of oolong tea on renal sympathetic nerve activity and spontaneous hypertension in rats

In a previous study, evidence was presented that oolong tea (OT) reduced abdominal fat accumulation in diet-induced obese mice. In the study presented here, we examined the sympathetic and cardiovascular effects of intraduodenal injection of OT in urethane-anesthetized rats and found that it suppressed renal sympathetic nerve activity (RSNA) and blood pressure (BP). In addition, pretreatment with the histaminergic H3-receptor antagonist thioperamide or bilateral subdiaphragmatic vagotomy eliminated the effects of OT on RSNA and BP. Furthermore, OT drinking for 14 weeks reduced BP elevation in spontaneously hypertensive rats. These results thus suggest that OT may exert its hypotensive action through changes in autonomic neurotransmission via an afferent neural mechanism. Moreover, we found that intraduodenal injection of decaffeinated OT lowered RSNA and BP as well as OT, indicating that substances other than caffeine contained in OT may function as effective modulators of RSNA and BP.

Potentiation of pulmonary reflex response to capsaicin 24h following whole-body acrolein exposure is mediated by TRPV1

Pulmonary C-fibers are stimulated by irritant air pollutants producing apnea, bronchospasm, and decrease in HR. Chemoreflex responses resulting from C-fiber activation are sometimes mediated by TRPV1 and release of substance P. While acrolein has been shown to stimulate C-fibers, the persistence of acrolein effects and the role of C-fibers in these responses are unknown. These experiments were designed to determine the effects of whole-body acrolein exposure and pulmonary chemoreflex response post-acrolein. Rats were exposed to either air or 3 ppm acrolein for 3 h while ventilatory function and HR were measured; 1-day later response to capsaicin challenge was measured in anesthetized rats. Rats experienced apnea and decrease in HR upon exposure to acrolein, which was not affected by either TRPV1 antagonist or NK(1)R antagonist pretreatment. Twenty-four hours later, capsaicin caused apnea and bronchoconstriction in control rats, which was potentiated in rats exposed to acrolein. Pretreatment with TRPV1 antagonist or NK(1)R antagonist prevented potentiation of apneic response and bronchoconstriction 24h post-exposure. These data suggest that although potentiation of pulmonary chemoreflex response 24h post-acrolein is mediated by TRPV1 and release of substance P, cardiopulmonary inhibition during whole-body acrolein exposure is mediated through other mechanisms.

The role of excitatory amino acids and substance P in the mediation of the cough reflex within the nucleus tractus solitarii of the rabbit

We hypothesized that cough evoked by mechanical stimulation of the tracheobronchial tree in the rabbit is primarily mediated by glutamatergic neurotransmission at the level of the caudal portions of the medial subnucleus of the nucleus tractus solitarii (NTS) and the lateral commissural NTS where cough-related afferents terminate, and that this reflex is potentiated by local release of substance P. To test our hypothesis, we performed bilateral microinjections (30-50 nl) of ionotropic glutamate receptor antagonists or substance P into these locations in pentobarbitone anaesthetized, spontaneously breathing rabbits. Blockade of NMDA and non-NMDA receptors by 50mM kynurenic acid abolished the cough reflex without affecting the Breuer-Hering inflation reflex or the pulmonary chemoreflex. Blockade of non-NMDA receptors using 10mM CNQX or 5mM NBQX caused identical effects. Blockade of NMDA receptors by 10mM D-AP5 strongly reduced, but did not abolish cough responses. Microinjections of 1mM substance P increased peak and rate of rise of abdominal muscle activity as well as cough number. These results are the first to provide evidence that ionotropic glutamate receptors, especially non-NMDA receptors, located within specific regions of NTS are primarily involved in the mediation of cough evoked by mechanical stimulation of the tracheobronchial tree in the rabbit. Present findings on substance P cough-enhancing effects extend previous observations and are relevant to the tachykinin-mediated central sensitization of the cough reflex. They also may provide hints for further studies on centrally acting antitussive drugs.

Involvement of the histaminergic system in renal sympathetic and cardiovascular responses to leptin and ghrelin

Previous studies have demonstrated that histamine affects blood pressure (BP) in anesthetized rats. Here, we examined the effects of lateral cerebral ventricular (LCV) injection of various doses of histamine on renal sympathetic nerve activity (RSNA) and BP in anesthetized rats. LCV injection of a low dose of histamine (0.0001nmol) suppressed RSNA and BP. Conversely, a high dose of histamine (100nmol) elevated both RSNA and BP. Moreover, inhibiting effects of a low dose of histamine were eliminated by LCV pre-injection of thioperamide, an antagonist of histaminergic H3-receptor, and accelerating effects of a high dose of histamine were abolished by LCV pre-injection of diphenhydramine, an antagonist of histaminergic H1-receptor. Thus, these evidences suggest that central histamine affects RSNA and BP via histaminergic receptors. In addition, we examined a role for histaminergic system in cardiovascular modulators such as leptin and ghrelin. The LCV pre-injection of thioperamide clearly blocked suppressing effects of ghrelin on RSNA and BP. The LCV pre-injection of diphenhydramine also blocked elevating effects of leptin. Therefore, these results suggest that leptin and ghrelin might affect RSNA and BP by mediating central histaminegic H1- and H3-receptors, respectively.

Effective antitussives for the cough patient: an unmet need

Chronic cough is a prevalent symptom in the community but it is likely that only a small fraction of chronic coughers seek medical attention. Chronic cough can be controlled by addressing the 'cause' of the cough, but not all cough is controlled using this approach; an 'idiopathic' cough or cough of unknown aetiology is becoming more well-recognized. In these patients and in those whose cough has not responded to treatment of the cause(s), there is a lack of efficacious antitussive therapies ('non-specific' antitussives). Even in those whose cough is controlled by treatment of the cause, an efficacious antitussive for symptomatic relief would be useful for breakthrough symptoms. It is necessary to address the mechanisms underlying chronic cough, particularly the process of sensitization, both peripherally and centrally, that is the basis of chronic cough; such a process may persist even in the absence of the initiating event that first induced the cough. Currently-available antitussives in both acute and chronic cough are not very effective. Novel targets that may result in effective antitussives have been identified and with the development of clinical tools to measure cough accurately and reliably (e.g. cough counts and cough-specific quality-of-life questionnaires) and the evoked cough response (e.g. citric acid or capsaicin challenges), clinical trials should be performed. The chronic cough population is clinically heterogeneous but is characterized by an enhanced cough reflex; this should be the target population for study. Patients with chronic cough are in desperate need of effective antitussives that can be used either on demand or on a long-term basis.

Plasticity of peripheral mechanisms of cough

The cough reflex pathway is characterized by a remarkable plasticity often resulting in a persistent and uncontrollable urge to cough during airway inflammation. In many instances cough becomes up regulated to the extent that ceases to fulfill its defensive role in protecting the airways. The exact mechanisms underlying this plasticity are unknown and likely involves a variety of factors influencing the function of the peripheral and central nervous system. This review outlines the evidence of increased cough sensitivity during airway disease. This is followed by a discussion of the peripheral mechanisms involved including the potential role of inflammatory mediators, neutrophins and changes in the airway mucosal structure. A greater understanding of the mechanisms leading to enhanced cough should lead to the development of more effective therapeutic strategies.

Measurement of cough

Cough is one of the most common symptoms encountered by clinicians particularly when it is persistent. Assessment of cough is essential for determining treatment outcomes, testing new therapies and for study of pathophysiology and mechanisms. There are new tools for measuring different aspects of cough. Apart from the patient's subjective assessment of cough severity using clinical scores, the impact of cough on quality of life can be measured. Direct measurement of cough counts is now possible by using portable devices, but how the counts relate to severity is unclear. A measure of cough intensity is required. Cough reflex measured by response to inhaling citric acid or capsaicin provides a measure of cough sensitivity that may be related to cough severity. In many intervention studies of cough, the response has usually been measured in terms of physician assessment of cough and its changes in cough reflex. How the information obtained between these different measurements interrelate needs to be determined.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

Capsaicin cough sensitivity in bronchiectasis

BACKGROUND

Bronchiectasis is a suppurative airway disease characterised by persistent cough and sputum production associated with bronchial dilatation. A study was undertaken to determine whether cough sensitivity is increased in bronchiectatic patients.

METHODS

Twenty two patients with bronchiectasis and 20 healthy non-smoking controls matched for age and sex were recruited into the study. Quality of life (Leicester Cough Questionnaire score), total cough symptom score, and extent of bronchiectasis on HRCT scans were recorded. Cough sensitivity was assessed using incremental inhalation of capsaicin concentrations; the concentration at which 5 or more coughs occurred (C5) was recorded.

RESULTS

Patients with bronchiectasis had increased sensitivity to capsaicin compared with controls (mean (SE) log10 C5 1.22 (0.20) v 1.89 (0.21); p<0.03). Capsaicin sensitivity correlated positively with the Leicester Cough Questionnaire score (r = 0.64; p = 0.005) and inversely with the total cough symptom score (r = -0.58; p = 0.004), but not with the extent of the disease. It also correlated with forced expiratory volume in 1 second (FEV1) in litres (r = 0.58; p = 0.005) but not with FEV1 % predicted. Capsaicin sensitivity was not related to the presence of infected sputum or to corticosteroid or bronchodilator use.

CONCLUSIONS

: Patients with bronchiectasis have a sensitive cough reflex which reflects the severity of cough symptoms. A measure of cough severity could be part of health assessment for patients with bronchiectasis.

Extending the understanding of sensory neuropeptides

The tachykinins substance P and neurokinin A are present in human airways, in sensory nerves and immune cells. Tachykinins can be recovered from the airways after inhalation of ozone, cigarette smoke or allergen. They interact in the airways with tachykinin NK1, NK2 and NK3 receptors to cause bronchoconstriction, plasma protein extravasation, and mucus secretion and to attract and activate immune cells. In preclinical studies they have been implicated in the pathophysiology of asthma and chronic obstructive pulmonary disease, including allergen- and cigarette smoke induced airway inflammation and bronchial hyperresponsiveness and mucus secretion. Dual NK1/NK2 or triple NK1/NK2/NK3 tachykinin receptor antagonists offer therapeutic potential in airway diseases such as asthma and chronic obstructive pulmonary disease.

Anatomy and Neurophysiology of the Cough Reflex

OBJECTIVES

To describe the anatomy and neurophysiology of the cough reflex.

METHODS

A review of the literature was carried out using PubMed and the ISI Web of Knowledge from 1951 to 2004. Most of the referenced studies were carried out in animals

CONCLUSIONS

Studies carried out in animals provide suggestive but inconclusive evidence that C-fibers and rapidly adapting receptors (RARs) arising from the vagus nerves mediate coughing. Recent studies also have suggested that a vagal afferent nerve subtype that is not readily classified as a RAR or a C-fiber may play an important role in regulating cough. Afferent nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature also likely play a less essential but important accessory role in regulating cough. The responsiveness and morphology of the airway vagal afferent nerve subtypes and the extrapulmonary afferent nerves that regulate coughing are described.

Synergistic interactions between airway afferent nerve subtypes regulating the cough reflex in guinea-pigs

Cough initiated from the trachea and larynx in anaesthetized guinea-pigs is mediated by capsaicin-insensitive, mechanically sensitive vagal afferent neurones. Tachykinin-containing, capsaicin-sensitive C-fibres also innervate the airways and have been implicated in the cough reflex. Capsaicin-sensitive nerves act centrally and synergistically to modify reflex bronchospasm initiated by airway mechanoreceptor stimulation. The hypothesis that polymodal mechanoreceptors and capsaicin-sensitive afferent nerves similarly interact centrally to regulate coughing was addressed in this study. Cough was evoked from the tracheal mucosa either electrically (16 Hz, 10 s trains, 1-10 V) or by citric acid (0.001-2 m). Neither capsaicin nor bradykinin evoked a cough when applied to the trachea of anaesthetized guinea-pigs, but they substantially reduced the electrical threshold for initiating the cough reflex. The TRPV1 receptor antagonist capsazepine prevented the increased cough sensitivity induced by capsaicin. These effects of topically applied capsaicin and bradykinin were not due to interactions between afferent nerve subtypes within the tracheal wall or a direct effect on the cough receptors, as they were mimicked by nebulizing 1 mg ml(-1) bradykinin into the lower airways and by microinjecting 0.5 nmol capsaicin into nucleus of the solitary tract (nTS). Citric acid-induced coughing was also potentiated by inhalation of bradykinin. The effects of tracheal capsaicin challenge on cough were mimicked by microinjecting substance P (0.5-5 nmol) into the nTS and prevented by intracerebroventricular administration (20 nmol h(-1)) of the neurokinin receptor antagonists CP99994 or SB223412. Tracheal application of these antagonists was without effect. C-fibre activation may thus sensitize the cough reflex via central mechanisms.

Activation of the nociceptin/orphanin FQ receptor reduces bronchoconstriction and microvascular leakage in a rabbit model of gastroesophageal reflux

1. Nociceptin/orphanin FQ (N/OFQ) is the endogenous peptide ligand for a specific G-protein coupled receptor, the N/OFQ peptide receptor (NOP). The N/OFQ-NOP receptor system has been reported to play an important role in pain, anxiety and appetite regulation. In airways, N/OFQ was found to inhibit the release of tachykinins and the bronchoconstriction and cough provoked by capsaicin. 2. Here we evaluated the effects of NOP receptor activation in bronchoconstriction and airway microvascular leakage induced by intraesophageal (i.oe.) hydrochloric acid (HCl) instillation in rabbits. We also tested the effects of NOP receptor activation in SP-induced plasma extravasation and bronchoconstriction. 3. In anesthetized New Zealand rabbits bronchopulmonary function (total lung resistance (R(L)) and dynamic compliance (C(dyn))) and airway microvascular leakage (extravasation of Evans blue dye) were evaluated. 4. Infusion of i.oe. HCl (1 N) led to a significant increase in bronchoconstriction and plasma extravasation in the main bronchi and trachea of rabbits pretreated with propranolol, atropine and phosphoramidon. 5. Bronchoconstriction and airway microvascular leakage were inhibited by N/OFQ (3-30 microg kg(-1) i.v.) in a dose-dependent manner. The NOP receptor agonist [Arg14,Lys15]N/OFQ mimicked the inhibitory effect of N/OFQ, being 10-fold more potent, UFP-101, a peptide selective NOP receptor antagonist, blocked the inhibitory effects of both agonists. 6. Under the same experimental conditions, N/OFQ and [Arg14,Lys15]N/OFQ did not counteract the bronchoconstriction and airway microvascular leakage induced by substance P. 7. These results suggest that bronchoconstriction and airway plasma extravasation induced by i.oe. HCl instillation are inhibited by activation of prejunctional NOP receptors.

Dose-dependent effects of l-carnosine on the renal sympathetic nerve and blood pressure in urethane-anesthetized rats

The physiological function of l-carnosine (β-alanyl-l-histidine) synthesized in mammalian muscles has been unclear. Previously, we observed that intravenous (IV) injection of l-carnosine suppressed renal sympathetic nerve activity (RSNA) in urethane-anesthetized rats, and l-carnosine administered via the diet inhibited the elevation of blood pressure (BP) in deoxycorticosterone acetate salt hypertensive rats. To identify the mechanism, we examined effects of IV or intralateral cerebral ventricular (LCV) injection of various doses of l-carnosine on RSNA and BP in urethane-anesthetized rats. Lower doses (1 μg IV; 0.01 μg LCV) of l-carnosine significantly suppressed RSNA and BP, whereas higher doses (100 μg IV; 10 μg LCV) elevated RSNA and BP. Furthermore, we examined effects of antagonists of histaminergic (H1 and H3) receptors on l-carnosine-induced effects. When peripherally and centrally given, thioperamide, an H3 receptor antagonist, blocked RSNA and BP decreases induced by the lower doses of peripheral l-carnosine, whereas diphenhydramine, an H1 receptor antagonist, inhibited increases induced by the higher doses of peripheral l-carnosine. Moreover, bilateral lesions of the hypothalamic suprachiasmatic nucleus eliminated both effects on RSNA and BP induced by the lower (1 μg) and higher (100 μg) doses of peripheral l-carnosine. These findings suggest that low-dose l-carnosine suppresses and high-dose l-carnosine stimulates RSNA and BP, that the suprachiasmatic nucleus and histaminergic nerve are involved in the activities, and that l-carnosine acts in the brain and possibly other organs.

Substance P presynaptically depresses the transmission of sensory input to bronchopulmonary neurons in the guinea pig nucleus tractus solitarii

Substance P modulates the reflex regulation of respiratory function by its actions both peripherally and in the CNS, particularly in the nucleus tractus solitarii (NTS), the first central site for synaptic contact of the lung and airway afferent fibres. There is considerable evidence that the actions of substance P in the NTS augment respiratory reflex output, but the precise effects on synaptic transmission have not yet been determined. Therefore, we determined the effects of substance P on synaptic transmission at the first central synapses by using whole-cell voltage clamping in an NTS slice preparation. Studies were performed on second-order neurons in the slice anatomically identified as receiving monosynaptic input from sensory nerves in the lungs and airways. This was done by the fluorescent labelling of terminal boutons after 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbo-cyanine perchlorate (DiI) was applied via tracheal instillation. Substance P (1.0, 0.3 and 0.1 microM) significantly decreased the amplitude of excitatory postsynaptic currents (eEPSCs) evoked by stimulation of the tractus solitarius, in a concentration-dependent manner. The decrease was accompanied by an increase in the paired-pulse ratio of two consecutive eEPSCs, and a decrease in the frequency, but not the amplitude, of spontaneous EPSCs and miniature EPSCs, findings consistent with a presynaptic site of action. The effects were consistently and significantly attenuated by a neurokinin-1 (NK1) receptor antagonist (SR140333, 3 muM). The data suggest a new site of action for substance P in the NTS (NK1 receptors on the central terminals of sensory fibres) and a new mechanism (depression of synaptic transmission) for regulating respiratory reflex function.